The drilling of oil and gas wells is accomplished by using rotary drilling techniques. In these rotary drilling techniques, a drilling mud is circulated through the drill pipe, out the bit nozzles and it is returned to the surface via an annulus. The drilling mud serves to cool and lubricate the drill bit and drill pipe. In addition, the drilling mud develops a hydrostatic head to counterbalance formation pressures and removes cuttings from the borehole. Further, the drilling mud helps reduce the friction forces between the drill pipe and the borehole.
Even though a drilling mud is used during the subterranean drilling operations, drill bits and drill pipes still become lodged in subterranean formations from time to time. The term "stuck pipe" is conventionally used in the drilling industry to describe this problem. "Stuck pipe" is not limited to drill pipes, but includes drill pipes, drill collars, drill bits, stabilizers, reamers, casings, tubing and other items or tools which may become lodged in drilling operations. When a stuck pipe becomes lodged, it can not be raised, lowered or rotated. A stuck pipe increases the cost of drilling a well due to downtime of the drilling operation. In extreme cases, a stuck pipe may cause the abandonment of the drilling operation at a particular site, if the stuck pipe can not be economically released.
There are many causes of stuck pipe. The industry categorizes the causes as either differential or mechanical sticking.
Differential sticking occurs during most drilling operations. The hydrostatic pressure exerted by the drilling mud column is greater than the formation fluid pressure. In permeable formations, the mud filtrate flows from the borehole into the rock pores and builds up a filter cake. A pressure differential exists across the filter cake which is equal to the difference between the pressure of the mud column and the pressure of the formation.
When a pipe is central in the borehole, the hydrostatic pressure, due to the mud overbalance, acts in all directions around the pipe. If, however, the pipe touches the filter cake, the mud overbalance acts to push the pipe further into the filter cake, thus increasing the contact area between the pipe and the filter cake. Filtrate is still expelled and squeezed from the filter cake between the pipe and the formation thus shrinking the cake and allowing the pipe to penetrate further into the filter cake, thereby further increasing the contact area. If the pressure difference is high enough and acts over a sufficiently large area, the pipe may become stuck. Differential sticking usually occurs when the pipe has been motionless for a period of time, e.g., when making a pipe connection or during surveying.
The force required to pull differentially stuck pipe free depends on many factors including:
(1) The difference in the pressure between the borehole and the formation. Any overbalance adds to side forces which may exist due to the deviation of the hole.
(2) The surface area of the pipe embedded in the wall cake. The thicker the cake or the larger the pipe diameter, the greater this area generally is.
(3) The bond developed between the pipe and the wall cake is a very significant factor, being directly proportional to the sticking force. This can include frictional, cohesive and adhesive forces. It generally tends to increase with time, making it harder to pull the pipe free.
Differential sticking may be distinguished from other forms of sticking, such as mechanical sticking. Mud circulation is not interrupted as there is no obstruction in the borehole to stop the flow, as would be the case for pipe stuck due to hole bridging or caving. It is not possible to move or rotate the pipe in any direction.
When a pipe sticks, the driller usually tries to free it by mechanical movement, e.g., by pulling, jarring or, if the pipe was moving immediately prior to sticking, trying to move the pipe in the opposite direction. Frequently this fails to release the pipe and there is, of course, a limit to the force which can be applied, since too much force could pull the pipe apart and make the situation worse.
If the pipe remains stuck, it is then sometimes the practice to apply a pipe release agent, commonly called a "spotting fluid". These spotting fluids are chemically active mixtures, which may be oil or water based, which are placed over the stuck region in an attempt to free the pipe, if mechanically working the pipe fails to release the pipe. These spotting fluids are believed to act by attacking the mud filter cake. They are positioned in the borehole by pumping the spotting fluid down the hole to the stuck region in the form of a slug, also known as a pill. The pill generally contains sufficient material to cover the stuck zone and extend slightly beyond the area of the stuck zone. Pills frequently are left to soak the cake until the pipe is free or attempts to free the pipe are abandoned.
Over the years, many spotting fluids and spotting methods have been developed to lubricate the area of the stuck pipe which has become lodged and to aid in the releasing of the drill bit from the formation, freeing a stuck drill pipe.
In one prior art stuck pipe treatment, hydrocarbon oils, such as diesel oil, are injected into the borehole in the area of the stuck pipe. The mixture of the diesel oil and drilling mud provides a low density, lubricating environment around the stuck point of the drill. Further, the hydrocarbon rises due to its buoyancy, which causes the removal of the mud around the stuck point and makes well bore pressure (P.sub.w).ltoreq.the formation pressure (P.sub.f) See, for example, U.S. Pat. No. 3,217,802 to Reddie.
Attempts have been made to improve the effectiveness of hydrocarbon based spotting fluids by adding additional components to the hydrocarbon based spotting fluids. For example, U.S. Pat. No. 5,671,810 to Hodge et al. adds a carboxylic acid or a polycarboxylic acid and a surfactant to a hydrocarbon spotting fluid; U.S. Pat. No. 4,436,638 to Walker et al., adds a propoxylated C.sub.18 -C.sub.32 alcohol and an oil soluble emulsifier to a hydrocarbon based spotting fluid; and U.S. Pat. No. 4,427,564 to Brownawell et al. adds a propoxylated lactone acid and an oil soluble emulsifier to a hydrocarbon based spotting fluid.
In a different approach, U.S. Pat. No. 2,900,026 to Trusheim discloses the addition of at least a 10 percent solution of hydrogen peroxide to a borehole having a stuck drill pipe to disintegrate the wall cake which causes the lodging to the drill pipe. In addition, this patent teaches that the addition of a small amount of caustic soda and soda will aid the action of the hydrogen peroxide.
Aqueous spotting fluids have also been previously used in the art. U.S. Pat. No. 4,466,486 to Walker discloses a method for freeing stuck drill pipes by adding a mixture of water or brine with a cellulose polymer or a polyether polymer as a spotting fluid. Further, other methods for freeing stuck pipes include using water-based spotting fluids, such as U.S. Pat. No. 4,230,587 to Walker, which discloses a mixture of water or brine with a mixture of polyethylene glycols as a spotting fluid; U.S. Pat. No. 4,494,610 to Walker, discloses a spotting fluid comprising a lower alcohol with another component, such as water, and optionally weighting agent; and U.S. Pat. No. 4,614,235 to Keener et al., discloses adding a mono or polyalkylene glycol ether to a water based spotting fluid.
Recently, acid and ester based spotting fluid have been prepared and used to release stuck pipes. In particular, a carboxylic acid, such as acetic acid as is disclosed by U.S. Pat. No. 5,247,992 to Lockhart; fatty acid alkyl esters containing spotting fluids are disclosed by U.S. Pat. No. 4,964,615 to Mueller et al.; and a spotting fluid which is non-toxic and comprises glycerol-phosphoric acid ester and a polyacyloxy polycarboxylic acid esters of mono and/or diglycerides is disclosed in U.S. Pat Nos. 5,002,672 and 5,127,475 to Hayes et al.
Finally, a multi-step spotting method is disclosed by U.S. Pat. No. 5,415,230 to Fisk, Jr. et al. In the first step of this patent, a spotting pill containing brine is pumped into the region of the stuck pipe and allowed to soak. A second spotting pill is pumped into the region of the stuck pipe and is also allowed to soak. The second spotting fluid can be any spotting fluid known to those skilled in the art. The spotting fluid is allowed to soak in place for an extended period or until the pipe becomes free.